The present invention relates to precision measurement systems in general, and more particularly to a distance or displacement measurement system using sonic or ultrasonic waves.
Distance measurement systems are known wherein a burst of sonic or ultrasonic waves is emitted by a transmitter, and the time taken by the waves to reach a receiver or be reflected back to the point of origin is measured such as to provide an indication of the distance between the transmitter and the receiver, or between the transmitter and the reflector.
Sonic or ultrasonic continuous wave systems are also known. In continuous wave systems, an emitter or transmitter produces a sonic or ultrasonic wave signal at a known frequency, and a receiver located at some distance away from the transmitter detects the sonic or ultrasonic wave. The change in phase relationship between the wave signal emitted by the transmitter and the wave signal received by the receiver, and the number of phase reversals (360.degree. phase shift) provides an indication of the relative displacement when either the transmitter or the receiver, or both, are moved.
One of the problems involved in utilizing sonic or ultrasonic waves to measure distances or relative displacement between a transmitter and a receiver is that the measurement of the distance or displacement is subject to considerable inaccuracies due to variations in the speed of sound resulting from changes in ambient temperature, atmosperic pressure, humidity, etc. For example, a 10.degree. C. change in temperature of the atmosphere causes a 1.7% change in the speed of sound, therefore a 1.7% error in the distance being measured, unless the change in temperature is taken into consideration. When it is desired to provide accurate positioning of a machine tool table, or of a robot arm, or when accurate measurements of displacements are required, sonic or ultrasonic wave technology is precluded, unless certain precautions are taken.
For example, either the environment in which the measuring system operates must be closely controlled, or some correction factors msut be introduced into the system for compensating for some of the known predictable effects of the environment. However, even with the use of costly accurate sensors for monitoring the actual environment, and extensive and complex circuitry to provide the proper amount of correction, it is rather difficult to achieve the accuracy and stability required for the positioning of components under the control of numerical control units (N/C units), as is presently done with shaft encoders, resolvers, synchros, inductrosyns, and linear glass scales.
The sonic or ultrasonic wave is a sinusoidal pressure variation of the air or fluid in which the sonic or ultrasonic wave is traveling at the speed of sound. The speed of sound in the air or fluid in which the wave is propagated depends in turn on the type of fluid, its pressure, and its temperature. A continuous sonic or ultrasonic wave has a wavelength .lambda. which is related to the speed of sound and the frequency of the wave according to the following equation: ##EQU1##
From this basic equation, it is clear that a change either in the frequency of the wave or in the speed of sound causes a change in the wavelength. In order to achieve accurate measurements of distance and displacement, a precisely known and stable wavelength is required. A constant wavelength can be obtained by changing the frequency of the wave as a function of the change in the speed of sound due to the effect of the environment.